Today, in information recording techniques, in particular, magnetic recording techniques, with the rapid development of IT industry, there are increasing demands for technological innovation. In magnetic disks to be mounted in HDDs or the like, due to the requirement for higher capacity, there are demands for a technique that can realize an information recording density of 40 Gbit/inch2 (1 inch being 25.4 mm) or higher.
Recently, glass substrates have been receiving attention as a substrate for a magnetic disk suitable for higher recording density. Glass substrates have higher rigidity than metal substrates, and are thus suitable for allowing an increase in the rotation speed of magnetic disk drives. Furthermore, since a smooth and flat surface can be obtained, the flying height of magnetic heads can be easily decreased. Thus, glass substrates are suitable for improving the S/N ratio of recording signals and for increasing the recording density.
Usually, a glass substrate for a magnetic disk is produced by grinding and polishing the surface of a glass disk having a predetermined size. Here, as the method for forming a glass disk having a predetermined size, for example, a method may be used in which a glass disk is cut from a glass material which has been formed into a planar shape by a float process.
Non-Patent Document 1 listed below describes a planar glass obtained by the float process. Furthermore, Patent Document 1 listed below describes a glass substrate obtained by forming a cut line in a glass plate using a cutter.
Patent Document 2 listed below describes a substrate for an information recording medium using the float process, a production method thereof, etc.
For reference, examples of prior art documents include JP-B-2785906, JP-A-H02-92837, JP-Y-S57-23452, JP-B-S55-6584, JP-B-H05-35095, and JP-B-S55-29019.    Patent Document 1: Japanese Patent Publication JP-B-2973354    Patent Document 2: International Publication WO 02/076675    Non-Patent Document 1: Sumio Sakuhana and two others, “Glass Handbook”, 1st ed., 8th printing, Asakura Shoten K.K., Nov. 20, 1985, pp. 412-413.